1. Field of the Invention
This invention relates to a system and method that utilizes an exhaust gas sensor to determine a smoke level of exhaust gases in the exhaust system of an internal combustion engine.
2. Description of Prior Art
Internal combustion engine such as diesel engines can produce exhaust particulate emissions (commonly referred to as “smoke”) which pollute the ambient air. An exhaust smoke sensor on-board the engine or vehicle could enable closed loop engine control systems to limit or minimize these emissions, and could diagnose the performance of emission controls (such as particulate filters) that are intended to reduce particulate levels in the exhaust gases. Laboratory instruments capable of real time smoke measurements exist but these analysers require windows in the exhaust pipe or a sampling system to transfer exhaust gas from the engine to the analyzer. A rugged sensor suitable for direct installation in an engine exhaust pipe is needed for on-board applications.
A number of researchers have studied approaches to smoke sensing in which electrodes are inserted into the exhaust flow. In one approach, the electrodes are used to detect the naturally occurring electrical charges of the soot particles in the smoke. For example, U.S. Pat. No. 4,485,794 describes a system in which a particulate level signal is provided by sensing charged particles with an electrically-passive annular electrode positioned in the exhaust stream. In another approach, a high voltage bias is imposed between a pair of electrodes and the flow of electrical current (due to the conductivity of the soot particles) is measured. For example, Society of Automotive Engineers (SAE) technical paper SAE 2004-01-2906 describes a particulate carbon sensor with a typical bias voltage of 1000 V and current measurement by means of a charge amplifier circuit. Both of these approaches are subject to measurement errors when soot particles accumulate on the electrode surfaces. Neither of these approaches has demonstrated the ability to measure the low smoke levels emitted by low emission, clean diesel engines.
Another type of sensor described in U.S. Pat. No. 6,6324,210 monitors the accumulation of soot particles on a non-conductive substrate between a pair of electrodes by measuring the resistance between the electrodes. The sensor must be regenerated periodically by heating it to burn off the accumulated soot particles; therefore it is not suitable for continuous real time measurements.